1. Field of the Invention
The invention relates to the field of a junction-photovoltage method and an apparatus for contactless determination of an electrical and/or physical parameter of a semiconductor structure comprising at least one p-n junction located at a surface. Particularly, the invention relates to a method and an apparatus for non-contact determination of sheet resistance and leakage current of a semiconductor structure comprising at least two p-n junctions located at the surface of a substrate.
2. Description of the Related Technology
It is generally known that a field effect transistor comprises a channel region controlled by a gate electrode, whereby the channel region is electrically contacted via source/drain regions located at opposite ends along the length of the channel. When scaling down device dimensions, as e.g. predicted by the International Technology Roadmap for Semiconductors (ITRS), more complex processes and structures are needed to construct source/drain regions which comply with the electrical, physical and geometrical requirements imposed.
Ideally a source/drain contact region should offer a low sheet resistance, a low contact resistance, a low leakage current, and has a limited extension into the substrate and towards the channel, such that the contact region can be fabricated in thin semiconductor surface layers without affecting the dimensions of the channel region. To this end, advanced process techniques such as cluster implant, plasma doping, non-laser melt annealing etc., can be applied allowing shallow implantation depth and high activation of the implanted species. Also additional doped regions, such as extension regions, halo and/or pocket implants are formed adjacent to the source/drain regions to limit the impact of the source/drain contact region on channel characteristics, such as the effective channel length and/or the threshold voltage.
Often multiple species are combined to form the source/drains as e.g. in recessed source/drain structures whereby a cavity created at the location of the source/drain regions is filled with carbon-doped silicon. Hence, instead of consisting of a single doped layer, these contact regions might comprise multiple, often opposite, doped layers. It thus becomes more challenging to determine in an accurate way the electrical properties of a complex contact region, in particular of the surface layers involved: sheet resistance, junction leakage current between opposed doped layers etc. as not all defects are eliminated by these advanced anneal processes.
Recently, junction photo-voltage (JPV) based tools have been introduced, which allows obtaining, in a non-physical contact way, the sheet resistance of junction isolated layers positioned near the surface of the substrate. The Junction Photo-Voltage (JPV) technique allows measuring the variation of the near-surface band bending or surface barrier of a p-n junction when being illuminated with varying intensity. In a non-contact JPV measurement technique, an electrode of the measurement apparatus is positioned adjacent to, but not physically contacting, the surface of the sample under study as to form a capacitor structure therewith. The resulting electrostatic interaction between the two plates, i.e. the electrode and the sample surface, can be observed via variations in the charge stored or in the change of the force applied. From such measurements the variation of the band bending or barrier at the surface of the sample can be deducted.
U.S. Pat. No. 7,019,513 describes a junction photo-voltage apparatus and method for simultaneously measuring the sheet resistance of an ultra-shallow (sub 50 nm) p-n junction and its leakage current. The p-n junction is a stack of an n-doped and p-doped layer formed on a substrate. In the following, the term “stack” is used as a synonym for a semiconductor structure. The basis of the measurement is to use photo-excitation of carriers in this junction and to monitor, in a spatially resolved manner, the generation/recombination and drift of these carriers with two electrodes, a first electrode at the center of the probe and a second electrode offset from the first electrode. The measurement probe disclosed thus consists out of two parts, a first, transparent circular, electrode with a radius R0, through which first electrode the p-n junction can be illuminated, and a second, arc-shaped, electrode with inner and outer radii R1 and R2 and angle θ, spaced apart from the first electrode.
The first electrode is used to probe the surface photo-voltage V1 in the illuminated area underneath the first electrode, while the second electrode is used to probe the surface photo-voltage V2 outside the illuminated area. When using a intensity modulated light beam with a penetration depth larger than the width of the depletion region of the junction but smaller than the sum of the width of the junction depletion region and the carrier diffusion length, a junction photo-voltage will be measured at the first and second electrodes, respectively a first voltage V1 [V] and a second voltage V2 [V].
These voltages probed are given by the following equations (1) (cp. equations 6, 7 and 8 in U.S. Pat. No. 7,019,513).
                                          V            1                    =                                                                                  q                  ⁢                                                                          ⁢                  Φ                  ⁢                                                                          ⁢                                      R                    S                                                                    k                  2                                            ⁡                              [                                  1                  -                                                            2                                              kR                        0                                                              ⁢                                                                                            I                          1                                                ⁢                                                  (                                                      kR                            0                                                    )                                                ⁢                                                                              K                            1                                                    ⁡                                                      (                                                          kR                              0                                                        )                                                                                                                                                                                                          I                              0                                                        ⁡                                                          (                                                              kR                                0                                                            )                                                                                ⁢                                                                                    K                              1                                                        ⁡                                                          (                                                              kR                                0                                                            )                                                                                                      +                                                                                                            I                              1                                                        ⁡                                                          (                                                              kR                                0                                                            )                                                                                ⁢                                                                                    K                              0                                                        ⁡                                                          (                                                              kR                                0                                                            )                                                                                                                                                                          ]                                                                ⁢                                  ⁢                              V            2                    =                                    φ                              2                ⁢                π                                      ⁢                                                        2                ⁢                q                ⁢                                                                  ⁢                                                      Φ                    ⁢                                                                                  ⁢                                          R                      S                                                                                                  k                      3                                        ⁢                                          R                      0                      2                                                                      ⁢                                                                                                    I                        1                                            ⁡                                              (                                                  kR                          0                                                )                                                              ⁡                                          [                                                                                                    R                            1                                                    ⁢                                                                                    K                              1                                                        ⁡                                                          (                                                              kR                                1                                                            )                                                                                                      -                                                                              R                            2                                                    ⁢                                                                                    K                              1                                                        ⁡                                                          (                                                              kR                                2                                                            )                                                                                                                          ]                                                                                                                                                    I                          0                                                ⁡                                                  (                                                      kR                            0                                                    )                                                                    ⁢                                                                        K                          1                                                ⁡                                                  (                                                      kR                            0                                                    )                                                                                      +                                                                                            I                          1                                                ⁡                                                  (                                                      kR                            0                                                    )                                                                    ⁢                                                                        K                          0                                                ⁡                                                  (                                                      kR                            0                                                    )                                                                                                                                                                                          (        1        )            where k=√{square root over (RsGs+iωRsCs)}, Rs is the sheet resistance of the top layer of the junction [Ω/sq], Gs is the conductance of the junction at zero bias [Ω−1cm−2], Cs is the capacitance of the junction [Fcm−2], ω=2πf [rad] with f the intensity modulation frequency of the light beam [Hz], q is the elementary charge [C], Φ is the effective light flux propagating inside the semiconductor [Js−1], I and K are modified Bessel functions of respectively the first and second kind, and i is the imaginary unit, Ro radius of the first probe [cm], R1 and R2 are the inner and outer radii of the second probe [cm] while φ is the angle thereof [rad].
U.S. Pat. No. 7,019,513 further describes a method for determining from a test p-n junction the sheet resistance Rs, junction capacitance Cs and junction leakage current Js, the latter being proportional to the junction conductance Gs. First a junction capacitance calibration wafer with negligible leakage current and known sheet resistance Rsc is measured at a high intensity modulating frequency f1 to obtain the ratio (V1c/V2c) involving k=kc=√{square root over (2πifRscCsc)} leading to the determination of Csc, the capacitance of the junction of the calibration wafer. Then the test p-n junction is measured to obtain the ratios (V11/V21) and (V12/V22) at a high f1 and low f2 intensity modulation frequency, respectively. Combined with the expression V11/V1c, one obtains three equations in the three unknown test wafer parameters Rs, Gs and Cs from which one can extract these three values uniquely.
The method and apparatus described in U.S. Pat. No. 7,019,513 however show several shortcomings. One shortcoming is that the formulas used to determine the first and second voltages are rather complex and require considerable computation time. Another shortcoming is that the method only allows determining sheet resistance Rs, junction capacitance Cs and junction leakage current Js of a single p-n junction at the surface of a substrate, whereas advanced junctions might comprise multiple, often opposite, doped layers. Additionally, a reference measurement with a reference p-n junction is required to get a reasonable quantitative result. Further, the light beam involved has to be modulated at least two modulation frequencies.